Method for producing articles having high creep resistance



METHOD FOR PRODUCING ARTICLES HAVING HIGH CREEP RESISTANCE BruceFrederick Richardson, Ottawa, Ontario, Canada, assignor to CobaltChemicals, Limited, Cobalt, Ontario, Canada, a corporation of Canada NoDrawing. Application December S, 19:13, Serial No. 397,050

6 Claims. (Cl. 148-115) This invention relates particularly, althoughnot exclusively, to the production of parts for gas turbines, such asturbine blades and the like, which require high strength and highresistance to impact at both low and high temperatures as well as highresistance to corrosion, erosion and fatigue at high temperatures, highresistance to heat shock, and resistance to creep.

Two of the principal problems which have faced manu facturers of gasturbines are (1) the provision of an alloy which possesses satisfactoryhigh temperature properties, and (2) the economical fabrication of thealloy to form the turbine parts, such as by rolling, forging andmachining. Various attempts have been made to solve these problems bythe provision of various ferrous and non-ferrous alloys containingvarious combinations of elements and amounts thereof. So far as I amaware all of these various attempts have failed either because the alloythus provided did not possess one or more of the necessary properties orbecause the alloy did not possess one or more of these properties to asufficient degree. Alloys which have combined the desired properties forturbine blades and the like to the best degree have not been forgeableor machinable and have required precision casting which is a slow andexpensive method of fabrication. Furthermore, cast metals do not possessthe valuable properties which can be imparted to metals by mechanicallyworking them, as by rolling, forging, etc. Alloys which have beenforgeable have not possessed other necessary properties to the desireddegree, such for example as resistance to creep and erosion.

The present invention is founded, in part, upon the observation thatalloys theretofore proposed for turbine blades and the like whichpossess the greatest resistance to creep and erosion, and which alsopossess other desired. properties, are those which are not forgeable.The invention also is founded, in part, upon the observation that thepresence of a large amount of iron is detrimental in alloys for use attemperatures in excess of 1200 F., such as for turbine blades and thelike. As a result of these observations, the idea was conceived thatparts for gas turbines or superchargers, such as turbine blades,buckets, vanes, discs and the like, could be produced by selecting aforgeable non-ferrous alloy which possesses some or most of theproperties desired and, after mechanically working the alloy to thedesired shape, treating it to change its composition to a non-forgeablealloy and impart to its other desired properties without detrimentallyaffecting the desired properties possessed by the original alloy.Valuable properties also are imparted to the article as a result of themechanical working. By selecting an alloy having most or all theproperties desired in a turbine blade except, for example, highresistance to erosion and creep, a forged turbine blade may be producedhaving a final hard non-forgeable composition possessing high resistanceto erosion and creep as Well as the other properties desired.

As previously pointed out, in general, alloys for turbine blades whichhave possessed the greatest resistance to creep and erosion and whichalso have possessed other properties desired are those which have notbeen readily forgeable. A

States Patent somewhat similar situation exists with respect to someother articles which are not directly related to turbine parts but whichrequire some similar properties, such as great hardness, high strengthand high resistance to erosion at elevated temperatures. Thus, theproperties desired in certain cutting tools, extrusion dies and thelike, are often possessed to the greatest degree by alloys which are notreadily forgeable.

' In accordance with one aspect, the present invention contemplates aforged or otherwise mechanically worked non-ferrous article of the typeillustrated by gas turbine parts, such as turbine blades, certaincutting tools, extrusion dies and the like, fabricated from a selectedforgeable non-ferrous alloy having some of the properties desired and,which after mechanically working to the desired shape, has been treatedto change its composition to a non-forgeable alloy and to impart to itother desired properties without detrimentally affecting the desiredproperties possessed by the original alloy.

When a forged article is desired, 1 have found a suitable alloy for usein the practice of the present invention is an initially forgeablenon-ferrous alloy containing by Weight from about 17 to 27 percentchromium, about 5 to 15 percent nickel, about to percent cobalt, notmore than about 0.03 percent carbon, about 12 to 18 percent tungsten,and from about 1 to 4 percent titanium together with about 0.25 to 0.75percent silicon and 0.50 to 1.50 percent manganese, the balance beingimpurities. Iron may be present as an impurity in an amount notexceeding about 2 percent. in some instances, part or all of thetitanium may be replaced by other carbide or nitride forming elements,such as molybdenum, tantalum, columbium, etc., or the latter elementsmay replace part of the tungsten. By the practice of the presentinvention, this forgeable non-ferrous article ing, approximately to theshape of the article desired and thus impart to it valuable propertiesdue to the mechanical working. Then it may be machined, if desired, andtreated to precipitate hard compounds, as by nitriding, gas carburizingor pack carburizing, and thus change its composition to a non-forgeablealloy and impart to it other desired properties possessed by theoriginal alloy. If desired, the article then may be heat treated at thetemperature and for a period of time required to obtain a solid solutionof the original alloy after which it is air cooled rapidly to a blackcolor. In some instances, it is desirable finally to age harden thearticle by heating at a suitable temperature and time, such as about1,400 F. for about five hours.

For turbine blades and the like I presently prefer to employ anon-ferrous alloy containing by weight carbon as an impurity in amountnot exceeding 0.03 percent, about 0.50 percent silicon, about 1.00percent manganese, about 20 percent chromium, about 10 percent nickel,about 15 percent tungsten, about 2 percent titanium, and the balancecobalt, namely, about 50 percent cobalt, except for impurities. Iron maybe present as an impurity in an amount preferably not exceeding about 2percent.

As further illustrative of the invention, I have forged an alloy of thelast mentioned composition in one direction at a temperature of about2,250 P. to effect a reduction of about 69 percent and form a forgedarticle about one inch wide and two inches long and with a maximumthickness of one quarter inch and having a cross sectional sizeresembling that of a turbine blade. After forging, the article was packcarburized by heating for about 12 hours at a temperature of 2,150 F.with a conventional carburizing material. The article then was removedand heated in a furnace at about 2,200 F, for about 40 minutes that is,for a time sufficient to obtain a solid solution. It was then rapidlyair cooled to a black color. Finally, the article was subjected to anageing treatment by heating in a furnace at atemperature of about 1,400Fofor about hours.

The article as forged had a hardness of 29 Rockwell .C and aftercomplete treatment had a hardness of 41 Rockwell C. Microscopicexamination indicated complete penetration by carbon throughout thethickness of the article. Thus, by the above treatment, the propertiesof the original alloy were improved by the forging operation and itscomposition was converted from a forgeable alloy to a hard erosionresistant alloy in which the titanium and, to some extent some of thetungsten and chromium had combined with carbon to form hard compoundsdispersed in a strong matrix comprising essentially cobalt, nickel,chromium and some tungsten.

While the aflinity of carbon or nitrogen for chromium or tungstenusually is not as great as it is for titanium, the expression an elementcapable of forming hard compounds as used in the appended claims is notintended to exclude chromium or tungsten either with or without suchelements as titanium.

The present invention is not limited to the production of a forged ormechanically worked article from an initially forgeable alloy, whichafter being shaped, is treated to convert the alloy to a non-forgeablecomposition. In its broader aspect, the invention is applicable forproducing articles of the general type previously mentioned fromnon-ferrous alloys of a non-forgeable composition. Thus, suchnon-forgeable, non-ferrous alloys may be shaped by casting and thentreated in accordance with the invention to improve the properties ofthe alloy. Such non-forgeable, non-ferrous alloys should contain one ormore carbide or nitride forming elements. The cast alloy may besubjected, for example, to a carburizing operation to precipitatetherein hard compounds of carbon and one or more of the elements of theoriginal alloy. if desired, the article then may be heat treated aspreviously described in connection with the treatment of the initiallyforgeable non-ferrous alloy. One advantage of such treatment of a castalloy is to impart to the surface layer increased resistance to erosionwithout changing the characteristics of the core of-the article. if suchhard compounds were present in the alloy as cast they would bedistributed through the alloy and this, in many instances would not bedesirable.

The invention, also is applicable for producing articles from anon-ferrous alloy containing carbide or nitride forming elements whichis shaped solely by machining rather than by forging or by forgingandmachining. Such articles, after being shaped, may be treatedaspreviously described toprecipitate hard-compounds'therein followed, ifdesired, by heat treatment as previously described.

I claim:

1. '1' he method for producing articles having high strength at hightemperatures and high creep resistance which comprises forming aforgeable alloy consisting essentially of primary non-ferrous metals andabout 1 to 4 percent by weight of a secondary non-ferrous metal togetherwith about 0.25 to 0.75 percent silicon and about 0.50 to 1.00 percentmanganese by weight, said secondary metal having a substantially greaterafiinity for a selected non-metallic element than does any one of saidprimary metals, said primary metalscomprising about 35 to 55 percentcobalt, about 5 to 15 percent nickel, about 17 to 27 percent chromiumand. about 12 to 18 percent tungsten each by weight based upon theweight of the alloy, shaping the alloy to a desired shape, andsubsequentlytreating the alloy to introducctheselected non-metallicelement and form hard compounds of said secondary metal with saidnon-metallic element dispersed inamatrixcomprising essentially saidprimary metals thereby imparting high creep resistance to the alloy,said nonametallicclement about 5 to percent nickel, about 17 to 27percent chromium, and about 12 to 18 percent tungsten each by Weightbased upon the weight of the alloy, shaping the alloy to a desiredshape, and subsequently subjecting the alloy to a carburizing operationtofo'rm hard carbides of said secondary metal dispersed in a matrixcomprising essentially said primary metals thereby imparting high creepresistance to the alloy.

3. The method for producing articles having high strength at hightemperatures and high creep resistance which comprises forming aforgeable alloy consisting essentially of about 1 to 4 percent oftitanium, about 0.25 to 0.75 percent silicon, about 0.50 to 1.00 percentmanganese, about to 55 percent cobalt, about 5 to 15 percent nickel,about 17 to 27 percent chromium and about 12 to 18 percent tungsten eachby weight based upon the weight of the alloy, shaping the alloy to adesired shape, and subsequently subjecting the alloy to a carburizingoperation to form hard carbides of titanium dispersed in a matrixcomprising :essentially cobalt, nickel, tungsten and chromium therebyimparting high creep resistance to the alloy.

4. The method for producing articles having high strength at hightemperatures and high creep resistance which comprises forming aforgeable alloy consisting essentially of about 0.50 p,ercent silicon,about 1.00 percent manganese, about 20 percent chromium, about 10percent nickel, about 15 percent tungsten, about 2 percent titanium andabout percent cobalt each by weight basednpon the weight of thealloy,shaping the alloy to a desired shape, and subsequently subjecting thealloy to a carburizing operation to form hard carbides of titaniumdispersed in a matrix comprising essentially, cobalt, nickeLtungsten andchromium thereby imparting high creep resistance to the alloy.

5. The method for producing articles having high strength at; hightemperatures and high creep resistance which comprises forming aforgeable alloy consisting essentially of primary non-ferrous metals andabout 1 to 4 percent by weight ofa secondary non-ferrous metal selectedfrom the group consisting of molybdenum, tantalum, colurnbium andtitanium together withabout 0.25 to 0.75. percent silicon and about 0.50to 1.00 percent manganese by weight, said primary metals comprisingabout 35 to percent cobalt, about 5 to 15 percent nickel, about 17 to 27percent chromium and about 12 to 18 percent tungsten-each by weightbased upon the weight of the alloy,. shaping the alloy to a desiredshape, and subsequently treating the alloy to introduce a non-metallicelement selected from, the group consisting of carbon and nitrogen andform hard compounds of said secondary metal with said non-metallicelement dispersed in a matrix comprising essentially said primary metalsthereby imparting high creep resistance to the alloy.

6..The method asclaimed byclaim 5 wherein said non-metallic element iscarbon.

References Cited in the file of this patent UNITED STATES PATENTS2,242,254 Mansfield -May 20, 1941 2,677,631 -.Gresham et a1 May 4,1954

1. THE METHOD FOR PRODUCING ARTICLES HAVING HIGH STRENGTH AT HIGHTEMPERATURES AND HIGH CREEP RESISTANCE WHICH COMPRISES FORMING AFORGEABLE ALLOY CONSISTING ESSENTIALLY OF PRIMARY NON-FERROUS METALS ANDABOUT 1 TO 4 PERCENT BY WEIGHT OF A SECONDARY NON-FERROUS METAL TOGETHERWITH ABOUT 0.25 TO 0.75 PERCENT SILICON AND ABOUT 0.50 TO 1.00 PERCENTWEIGHT, SAID SECONDARY METAL HAVING A SUBSTANTIALLY GREATER AFFINITY FORA SELECTED NON-METALLIC ELEMENT THEN DOES ANY ONE OF SAID PRIMARYMETALS, SAID PRIMARY METALS COMPRISING ABOUT 35 TO 55 PERCENT COBALT,ABOUT 5 TO 15 PERCENT NICKEL, ABOUT 17 TO EACH BY WEIGHT BASED UPON THEWEIGHT OF THE ALLOY, SHAPING THE ALLOY TO A DESIRED SHAPE, ANDSUBSEQUENTLY TREATING THE ALLOY TO A DESIRED SHAPE, AND SUBSEQUENTLYTREATING THE ALLOY TO INTRODUCE THE SELECTED NON-METALLIC ELEMENT ANDFORM HARD COMPOUNDS OF SAID SECONDARY METAL WITH SAID NON-METALLICELEMENT DISPERSED IN A MATRIX COMPRISING ESSENTIALLY SAID PRIMARY METALSTHEREBY IMPARTING HIGH CREEP RESISTANCE TO THE ALLOY, SAID NON-METALLICELEMENT BEING SELECTED FROM THE GROUP CONSISTING OF CARBON AND NITROGEN.